Refrigerant circulating system with multistage compressor



y 9 194% M. G. SHOEMAKER 2,477,993

REFRIGERANT CIRCULATING SYSTEM WITH MULTISTAGE COMPRESSOR Filed April 29, 1944 2 Sheets-Sheet 1 M. G. SHOEMAKER REFRIGERANT CIRCULATING SYSTEM WITH MULTISTAGE COMPRESSOR July 26, 1949.

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REFBIGEBAN T CIROULATIN G SYSTEM WITH MULTISTAGE COMPRESSOR Malcolm G. Shoemaker, Abington, Pa., asslgnor, by mesne assignments, to Philco Corporation,

Philadelphia, Pa., vania a corporation of Pennsyl- Application April 29, 1944, Serial No. 533,354

comm. (Cl. 62-115) This invention relates to refrigerating apparatus and particularly to an improved refrigerant circulating system. More specifically the invention has reference to an improved multi-stage motor-compressor unit particularly adapted'ior use in refrigerators of a type aflording the low temperature conditions required for frozen foods. Neither the single nor multi-stage motor-compressor units heretofore known in the refrigerator art are well adapted for either practical or efflcient operation in the newer types of refrigerator in which at least one compartment is maintained at temperatures ranging, for example, from to -20, necessitating a compressor having a wide capacity range. Because of the relatively large volume of refrigerant which must be handled by the compressor at such low operating temperatures, a single-stage unit having an adequate capacity range would necessarily be more bulky than the ordinary single-stage unit now in common use in standard types of refrigerators and would therefore be undesirable for use in a home refrigerator wherein conservation of space is an important factor. Also, a single-stage unit of this size would require a motor having a relatively high H. P. rating to accommodate the extreme peak load experienced at the high initial pressures, and the motor would then operate at low efflclency at the reduced normal running pressure when the motor load would correspond to only a fraction of its capacity.

The introduction into the art of multior twostage compressors has afl'orded an increased B. t. u. capacity range in a unit comparable in size to that previously used in home" refrigerators having the conventional freezing and food storage compartments. However, use of this type of unit results either in detrimental motor overload at the aforesaid high initial pressures or entails the use of a more powerful motor. with attendant lower emciency of operation at-the reduced normal running pressures.

An important object of the present invention is to provide an improved multi-stage motor-compressor unit more adequately filling the requirements of the advanced types of domestic refrigorator.

- Another important object of the invention is the provision of such senator compressor characterized by practical and emcient operation over a relatively wide range of suction pressures.

' It is also an important object to provide in a refrigerant circulating system a motor-compressor unit for low temperature evaporator opera- 2 tion, having a substantially constant B. t. u. capacity over a wide range of suction pressures so as to present a substantially constant motor load regardlessof suction pressure fluctuation.

A further object of the invention is to provide a motor-compressor unit automatically responsive to a pre-selected suction pressure value for operation selectively as a single or multi-stage I view of a hermetically sealed unit.

A more specific object of the invention is to provide a multi-stage motor-compressor unit of a sine suitable for domestic refrigerator use, said unit including a motor of relatively low H. P. and having a wide suction pressure capacity range throughout which said motor is operative at substantially peak eiilciency.

. A still more specific object of the invention is to provide a two-stage motor-compressor unit in which a substantially constant pressure is delivered from the low-pressure pump to the highpressure pump over at least a portion of the suction pressure capacity range of the unit.

A still further object of the invention is to provide a motor-compressor unit adapted to operate as a single-stage. unit at high suction pressures and as a two-stage unit at lower suction pressures.

Another object of the invention is to provide a motor-compressor unit having means responsive to suction pressure for automatically increasing the B. t. u. capacity of said unit.

In the attached drawings:

Figure 1 is a diagrammati illustration of a. refrigerator system embodying the invention:

Figure 2 is a fragmentary vertical sectional motor-compressor unit embodying the invention;

Figure 3 is a fragmentary sectional view on the line 3-3 of Figure 2;

Figure 4 is a fragmentary sectional view on t line H of Figure 2;

Figure 5 is a fragmentary-sectional view on a considerably enlarged scale taken on the line b-t of Figure t;

Figure 6 is a graph illustrating an operating gycle of a preferred embodiment of the invenion;

Figure 7 is a fragmentary diagrammatic view of a modified form of the invention; and

Figure 8 is a graph illustrating an operating" cycle of a modified embodiment of the invention.

Referring to the drawings, and more particularly to Figure 1, the refrigerant circulating sys-{g tem i0 therein illustrated includes a conventional; condenser ii from which liquid refrigerant flows through a capillary tube i2 into an evaporator i3 of either the series or parallel feed type, as desired. Gaseous refrigerant passes from the evaporator l3 through a suction line l4 into the compressor i5 where it is compressed and from which it passes through a tube l6 into the condenser ll, thereby completing the cycle. .The capillary tube 82 and suction tube i4 may be in heat exchange relation as illustrated.

The compressor 15, as more clearly illustrated in Figure 2, is of the multi-stage (in this instance two-stage) hermetically sealed type and includes a shell or housing comprising a pair of opposed bowl or dome-shaped members 21 and 22 sealed together about their abutting peripheral flanges 23 and 24 as by welding, indicated at 25. Legs 26 (Figure 1) are provided for mounting the shell within the machine compartment of a refrigerator (not shown).

Mounted within shell 20 upon a plurality of resilient supports, of which one is indicated at 21, is a main frame member 28 provided with a central vertical bore forming a bearing 38 for a shaft 3|. Frame 28 is also provided with upstanding wall portions 32 for supporting the stator 33 of an electric motor 34. The rotor of motor 34, located within the stator and generally designated by the numeral 36 (Figure 1), is rigidly secured to the upper end of shaft 3i.

Shaft 3| has an integral lower extension or eccentric 31 having its vertical axis offset horizontally relative to the normal axis of shaft 3i. This eccentric is adapted to cooperate with a scotch yoke" 38 in reciprocating a pair of horizontally opposed integral pistons 39 and 48 in high and low pressure cylinders 4| and 42 in the lower portion of frame member 28, said piston-cylinder assemblies constituting respectively elements of high and low pressure pumps 43 and 44. The yoke assembly comprises a hearing element 45 which moves with the eccentric 31 and slides transversely in the yoke member 35 of which member the pistons are a part.,

The cylinder of the low pressure pump 44 comprises a cylinder head 41 which is secured to frame 28 as by machine screws 49 (Figure 4). Cylinder head 41 is provided with inlet apertures 50 normally closed by a spring leaf valve 5i fastened to the inner surface of head 41, as by screws 52, and comprising a pair of legs 53, 53 depending from a transverse portion 54 through which portion pass the screws 52. To prevent oil from entering these apertures from the crankcase 22a formed by the lower portion 22 of shell 20, an oil-tight hood 55 is secured thereover as by screws 55a. From the hood 55 an open ended tube 56 extends upwardly to a point sufficiently distant from the oil level 51 to prevent oil from entering the open end to find its way into pump 44. If desired a baffle 58 may be fitted around the tube to prevent oil from splashing or foaming into its open end.

Still with reference to Figure 2 it may be seen that the cylinder of the high-pressure pump 43 comprises a valve plate 60 and cylinder head El secured to frame 28 as by machine screws 62. Valve plate fits snugly in a recess 83 in the head BI, and gaskets 64 on both sides of the outer edges of the plate seal the cylinder against leakage of refrigerant. Valve plate 60 is provided with outlet apertures 65, and these apertures are normally closed by a resilient disc valve 66 snugly fitting into a recess 61 in cylinder head 8! and resiliently held in such position by a coil spring 88 seated in a recess 69 also inthe head 6|. A channel 10 through the integral pistons 88 and 40 provides communication from cylinder 42 to cylinder 4i, said channel at the end adjoining the cylinder 4| terminating in a circular groove II which is normally closed by a resilient disc valve 12 secured to the piston as by a screw 14.

A plurality of recesses 15 (Figure 3) are provided in the walls of the recess 6'! in head 6! to permit passage of gases from beneath disc valve 66 into said recess. Open communication between recess 61 and discharge tube is is afforded by a tube 150, muilier l8, and tube ll, one end of tube 15a being fitted into an outlet aperture 18 in head 8|, and the other end being connected to the muffler 16 from which the tube ll extends to and through the shell 20 by way of an air-tight leak-proof Joint shown at 18. One end of the discharge tube It is secured to the outer end of tube l1, and the other end is attached to one end of the condenser tube 88 so as to afford open communication between the recess 51 in cylinder head SI and the condenser H. Tube Il may be extended to loop around the frame 28 prior to its attachment to shell 28, thereby to reduce vibration at the point of attachment. Mufller 18 may be cast integrally with the frame 28 or cylinder head iii, if desired.

While any suitable lubricating system may be incorporated in the unit, an impeller type of oil pump has been shown and includes a casting 8| hollowed out, as at 82, to form a recess or cavity for reception of an impeller 88, said impeller being attached to the lower end 84 of an integral small diameter axial extension 85 of shaft 3!, which extension passes through a central bearing 88 in the casting 8|. A cover plate 81 encloses the recess and together with the casting 8| is-fastened to the lower portion of the frame 28 as by screws 88. Open communication between crankcase 22a and recess 82 is provided by anaperture 88 in the cover plate 87. A passage 90 in the head has one end in communication with recess 82 at a point remote from aperture 89 and its other end in registry with an axial bore 9| in the shaft 31, which bore extends upwardly to a radial bore 82 having connection with a spiral channel 83 about the periphery of shaft 3|. Another radial bore 94 in eccentric 81 provides a means for lubricating the scotch yoke 38.

When the motor 34 is rotating the impeller 83 sucks oil from crankcase 22a into recess 82 through aperture 89 and forces it by way of passage ,90 upwardly through the axial bore 98 and through the radial bore 92 into channel 83 to lubricate the main shaft bearing 30. Oil also passes through radial bore 94- to lubricate the scotch yoke 38. From the top of spiral channel 93 oil flows into a peripheral channel (not shown) from which it overflows and finds its .way downwardly over the frame and pumps, back into the crankcase to repeat the cycle.

The above described compressor mechanism is in accordance with well-known principles in the compressor art and the invention does not lie in such mechanism, per se, except in so far as associated with the following structure.

In accordance with the present invention and as clearly illustrated in Figure 5, there is provided in cylinder head 41 of low pressure pump 44 a pressure-responsive valve, generally indicated by reference numeral Hill. This novel valve means, as will be fully brought out, is adapted to render one of said pumps ineifective under certain conditions of operation. In the broader aspect, valve means I may take any one of several known forms. However, the valve preferably is of. the bellows type and comprises a base member IOI secured in an aperture I02 in the head 41 as by a threaded extension I03. An intermediate peripheral nut-shaped flange or projection I04 is adapted to receive a wrench for effecting such attachment. A gasket I05 is interposed between the flange and cylinder head 41 to effect an air-tight seal at that juncture,and a flange I06 of the hood 55 also extends upwardly beneath the flange I04 to provide additional connection between the hood and the cylinder-head. Extending in a direction opposed to that of threaded extension I03 is an extension I0'Ihaving threaded thereon a dome or cap I08 provided with apertures I09 and supporting the pressure-responsive element H0 of the valve. Base member IOI has an axial bore or aperture I I I providing communication with the interior of shell 20 through apertures I00. A valve seat H2 on the face H3 of projection I01 surrounds the mouth H4 of aperture III and is adapted to cooperate with plate II5 of element H0 to control communication between shell 20 and cylinder 42.

The valve plate II5 has an annular flange I I6 to which is secured, as by welding or other suitunder the above conditions, will be equalized throughout at, say, approximately 84 pounds and the pressure-responsive .valve I00 will be open.

When it is desired to initiate operation of the shell interior by reason of open valve I00 so there able method, one end of a bellows III, the other end of the bellows being similarly secured to a peripheral flange H8 of an inwardly projecting centrally disposed member H9 thereby forming within the bellows an air-tight cell I20 which may be sealed at atmospheric or other desired pressure. A coil spring I2I is disposed around member H9 within the cell I20 and normally holds the plate I I5 and flange I I8 in spaced relation. For supporting element H0 in a centrally disposed position relative to the dome I08, member H9 is provided with a central bore or recess I22 which receives one end I23 of a. screw I24 threaded through an inwardly extending internally threaded boss I24a of dome I08. By turning the screw I24 inwardly or outwardly element IIO may be set at some predetermined pressureresponsive'value which the external pressure on the bellows would have to exceed to move plate H5 from its seat H2 to establish communication between cylinder 42 and shell 20. A look nut I25 is provided on the screw externally of dome I08 for fixing the valve in its selected pressureresponsive adjustment. The dome or cap prevents surging of large quantities of oil through .,ment H0 of the valve exceeds pounds, bellows ill will collapse and unseat valve plate H3 from lts seat H2 and open communication of cylinder is no pumping action by actuation of piston 40, and the operation of the unit is conventional single-stage, as indicated by line I26 of Figure 6. when piston 39 moves in a direction outwardly of its cylinder 4|, disc valve I2 is-moved from its 15a, muffler I6, and tubes I1 and I6 into condenser II.

Continued operation of the unit as single-stage draws gaseous refrigerant from the evaporator through tube I4 and into shell 20 at decreasing suction pressures as the temperature of the evaporator is lowered. When the pressure within the shell decreases to 30 pounds gauge, pressure responsive element I I0 closes aperture II and re-- frigerant then will enter cylinder 42 only through M with the interior of shell 20 through aperture HI. When the pressure upon element H0 is less than 30 pounds, plate H5 is urged against its seat by the element to close the aforesaid communication between the cylinder and shell 20.

As the system illustrated in Figure l is of the continuously open capillary type, the pressure,

aperture 50, by way of tube 56 and hood 55, the valve 5I being unseated or opened on the suction stroke of piston 40 and closed on its compression stroke. It will be understood, of course, that the pressure value of 30 pounds gauge has been selected only by way of example, and that by preselection the valve I00 may be set at other desired pressure-responsive values by adjustment of screw I24.

As stated, valve I00 closes at 30 pounds pressure in shell 20, but at that value, immediately opens upon initiation of the compression stroke of pump 44 by reason of the increased pressure in cylinder 42, to thus vent the excess back into the shell 20. As the pressure within the shell drops below 30 pounds the valve I00 stays closed for longer periods, in other words, it stays closed until the pressure in cylinder 42 exceeds 30 pounds, at which value the valve opens to vent the excess back into the shell. This operation continues over a wide range of lowering pressures within shell 20 until pump 44 'is no longer able to increase the pressure received within cylinder 42 above 30 pounds. Thus a substantially constant weight of refrigerant is fed from cylinder 42 into cylinder 4| over a wide range of suction pressures, as represented by the horizontal line I21 in Figure 6. When pump M is no longer able to raise above. 30 pounds the pressure of the refrigerant received within its-cylinder 42 from shell 20, the valve I00 remains closed and continued operation of the unit is then conventional two-stage, as represented by line I28 in Figure 6.

With regard-to the above described operation of the unit it should be understood that upon the initial stroke of piston 30 there is no discharge pressure to overcome, excepting the weight of discharge valve 56, and it should be pointed out that the latent-heat content or the warm refrigerant prior to operation of the appsratus is approximately 60.5 B. t. u. per pound at 80, which means that a 1.5 cubic inch per stroke displacement pump with a 1750 R. P. M. motor, would displace approximately 1.5 cubic feet per minute by volume and (with Freon 12" having a density or 2.35 pounds per cubic foot) that.

approximately 210 B. t. u. per minute would be the starting rate of discharge into the condenser, said condenser being designed to dissipate a condenser output of approximately B. t. u, per minute at normal running pressures.

Because large condensers are uneconomical and unnecessary at normal operating pressures. it is customary to use small condensers in domestic size refrigerators, and such condensers are inadequate to dissipate the aforesaid large B. t. u. I

output of the compressor during the initial running period. Consequently the discharge pressure of pump 43, or that back pressure from the condenser which resists opening of the disc valve 66, increases rapidly from zero on the initial stroke of the pump to perhaps 350 pounds in as few as two minutes or even less, depending upon the R. P. M. rate of the motor. This pressure must be slightly exceeded by the pressure in cylinder 4| before gas will start passing exhaust valve 66 into the condenser. In addition to reaching this high side pressure, it must overcome the exhaust valve pressure or weight, with a consequent further increase in pressure. At such high pressures it is well-known that volumetric efliciency is low.

Volumetric emciency of a pump is defined by the ratio between the volume actually pumped per revolution divided by the volume calculated from the bore and stroke. The volume actually pumped in refrigerator motor-compressor units is dependent upon the discharge pressure coupled with re-expansion of the small volume of gas still remaining inthe cylinder between the piston and cylinder head at the completion of the compression stroke. This small volume is under high pressure and as the piston returns on the suction stroke this gas expands and partially fills the cylinder chamber to decrease by this amount the quantity of gas that may enter the cylinder from the low pressure or suction side of the system, from shell as in the present instance. There are also other incidental losses such as those due to leakage past the piston back into the shell and from super-heating of the gas resulting from its contact with the heated motor, etc all tending to decrease the volumetric emciency. Thus it may be seen that it is extremely advantageous for efficient and economical operation to keep discharge pressures and consequently compression ratio as low as possible.

With the above in mind it will be understood that efiective operation of low pressure pump 63 8 in addition to operation of the high pressure pump during the initial operating period would discharge a much greater weight of gas into the condenser thereby increasing the discharge pressure more rapidly and to a much higher value, and would require a motor having a higher HP rating to overcome this increased load. Such a. motor would be increasingly inemclent as the pressures lowered, and extremely so at the low pressure encountered at l0 F. In this respect it may be stated that a particular requirement of all motor-compressor units is that the motor must have a capacity sufiicient to meet the relatively infrequent peak or maximum load. It should be understood too that if the unit were allowed to continue operation as a single-stage unit it would do so in a manner similar to a conventional single-stage compressor unit, but as the sizeand pump displacement of the single cylinder is necessarily small in domestic size units, its B. t. u, capacity at low suction pressures would be inadequate for the desired extremely low temperatures, such as -10, see line Mia, Figure 6.

Comparing the operation of the present unit with that of known single and two-stage motorcompressor units having a capacity substantially equal to a unit corresponding to the above described embodiment of the invention, it should be understood that each of the known types would require a motor having a high H. P. rating to accommodate the peak starting load, with a resultant greatly decreased operating efficiency at the reduced pressures incident to low temperature operation.

While it is impossible'to alter the above condition in single-stage operation, the present invention provides a two-stage construction wherein the starting loads are decreased to an extent equal to the capacity of the low pressure pump, with a resultant comparative decrease in H. P. requirement, and increased operating efliciency at low temperature operation. Let it be assumed that the reduced H. P. motor of the present invention be somewhat overloaded (but within its limits) under the pressures encountered at the initiation of operation as a single-stage unit, by the time valve we closes and the second stage has functioned intermittently, as described, the motor will have operated at substantially peak efficiency over a wide range of pressures, and there will be little loss of efilciency after initiation of conventional two-stage operation and continued operation to the desired low temperature.

It is evident from the above that the invention has provided an improved home size multistage motor-compressor unit characterized by efficient operation. over a wide range of suction pressures. It is evident also that a unit has been provided which is capable of automatically changing its operating'cycle from single to twostage, or vice versa, depending upon suction pressures. Further, during such change of cycle, it is capable of operating automatically at a B, t. u. capacity relatively constant over a wide range of suction pressures so as to present a constant motor load regardless of suction pressure fluctuation, and this while utilizing a minimum H. P, motor at substantially peak efliciency throughout the capacity range.

Figure 7 illustrates a modified form of the invention and includes in place of pressureresponsive valve its, a solenoid actuated valve are mounted over aperture ldia, in cylinder head sic, and having a valve plate 5c adapted to 'seat on an annular seat its surrounding aperture lute. A pressure-responsive switch use is adaptedto open and close a suitable electrical circuit 135 for actuating valve I38 to and from its seat 433. Switch we may be set to operate at any pro-selected suction pressure value by turning screw 36. A look nut I3? is provided to fix" such setting of the switch.

In operation of this form of the invention, switch i3i closes at high suction pressures to complete the circuit 635, thus actuating the solehold to draw valve plate 5a away from its seat we to open communication between cylinder 32 and casing 28. When the suction pressure has switch I operates to break circuit I35 to allow the valve plate to be moved, under pressure of spring I38, to its closed position on seat I33 to close the aforementioned communication of cylinder 42 with shell 20. The suction pressure valve is preferably set at a pressure value which, when the valve closes, will not increase the motor load of the two-stage operation to a value greater than "the starting load of the initial single-stage operation. The advantages of two-stage capacity range are thus acquired without the initial motor overload characteristic usually attending such units.

While it is true that the present invention as illustrated is particularly adapted for use in "home" size two-stage hermetically sealed motorcompressor units, its application should not be considered to be so limited as its principle may extend to large multi-stage industrial units of .similar type, large and small multi-stage units of the so-called open type, and is also applicable to the rotary type of compressor.

I claim:

1. In a refrigerant circulating system including an evaporator and a condenser, a two-stage compressor unit including a sealed housing having communication with said evaporator for receiving gaseous refrigerant therefrom, a low-pressure pump and a high-pressure pump mounted within said housing, means for actuating said pumps, said low-pressure pump being adapted to receive gaseous refrigerant from within said housing to compress said refrigerant and to pass it into said high-pressure pump to be further compressed and discharged 'into said condenser wherein it is liquefied and subsequently returned to said evaporator, and means responsive to high pressures within said housing for rendering said low-pressure pump ineffective whereby said unit may operate as a single-stage unit.

2. In a refrigerant circulating system including an evaporator and a condenser, a two-stage motor-compressor unit including a sealed housing having communication with said evaporator for receiving gaseous refrigerant therefrom, a lowpressure pump and a high-pressure pump mounted within said housing, a motor for actuating said. pumps, said low-pressure pump being adapted to receive gaseous refrigerant from within said housing to compress said refrigerant, and to pass it into said high-pressure pump to be further compressed and discharged into said condenser wherein it is liquefied and subsequently returned to said evaporator, and means responsive to pressures within the housing for rendering said lowpressure pump ineffective, whereby said unit may operate as a single-stage unit, said means being automatically responsive to reduced housing pressures for rendering said low-pressure pump effective, whereby said unit may operate at a relatively constant capacity over a wide range of suction pressures so as to present a relatively constant load upon the motor regardless of suction-pressure fluctuation.

3, In a refrigerant circulating system including an evaporator and a condenser, a two-stage mopression and subsequent discharge into said condenser wherein it is liquefied and thereafter returned to said evaporator, and means responsive to pressure within the housing for rendering 'ineffective said low-pressure pump whereby during a portion of said unit operating cycle the unit may operate as a single-stage unit, said means being automatically responsive to reduced suction pressures for rendering said low-pressure pump operative whereby further operation of the unit is two-stage.

4. In a refrigerant circulating system a motor driven multi-stage compressor unit including a high-pressure pump, a low-pressure pump, and valve means for rendering the low-pressure pump'inefiective during a portion of an operating period of said unit, said valve means comprising a port-providing communication between the said low-pressure pump and the source of gaseous refrigerant, a valve element controlling said port and arranged so that internal pump pressure tends to open the valve, pressure-responive means actuatable by pressure of said gaseous refrigerant to open the valve, and means for predeterminedly loading the valve against said opening pressures.

5. In a refrigerant circulating system a motor driven multi-stage compressor unit including a high-pressure pump, a low-pressure pump, a bypass port in the low-pressure pump operative when open to render the pump ineffective, and a valve controlling said port, said valve being responsive to pressure from within the pump to uncover the port, and means operatively connected to the valve for resisting with predetermined pressure the tendency of said internal pump pressure to uncover the port, and means responsive to the pressure prevailing at the suction side of the pump for uncovering the port independently of said internal pressure when said suction pressure exceeds said predetermined pressure.

tor-compressor unit including a sealed housing having communication with said evaporator for receiving gaseous refrigerant therefrom, a lowpressure pump and a high-pressure pump mounted within said housing, a motor for actuating said pumps, said low-pressure pump being adapted to receive refrigerant from within said housing, to compress said refrigerant, and to pass it into said high-pressure pump for further com- 6. A multi-stage compressor umt including a low and a high pressure pump, a by-pass port in the low pressure pump adapted when open to render the pump ineffective, a valve controlling the port and responsive to pressure from within the pump to uncover the port, and'means operatively associated with the valve and exerting thereon a predetermined pressure resisting the tendency of said internal pump pressure to uncover the port, said means being responsive to pressures at the suction side of the pump to open the valve when said suction pressure exceeds said predetermined pressure.

7. A multi-stage compressor unit including interconnected high and low pressure cylinders and pistons operating synchronously therein, a normal valve-controlled suction port in the low pressure cylinder, a by-pass port in the low pressure cylinder adapted when open to render the said cylinder inoperative, a valve controlling said bypass port and responsive to pressure from within the said cylinder to uncover the port, means for loading the valve to a predetermined opening pressure, and means operatively connected to the valve and adapted for actuation by fluid pressure pass port and responsive to pressure from within the said cylinder to uncover the port, means for loading the valve to a predetermined opening pressure, and means actuated by external pressure at the said suction port for opening the valve against said loading pressure.

9. In a closed refrigerant circulating system, a multi-stage compressor unit including interconnected high and low pressure cylinders and pistons operating synchronously therein, an evil orator connected to the suction side of the low pressure cylinder, a condenser connected to the discharge side of the high pressure cylinder, and means for connecting the condenser with the evaporator to complete the refrigerant circuit, a check valve in the connection between the evaporator and the low pressure cylinder, a bypass for said check valve, avalve controlling said by-pass, said valve being responsive to the internal pressure of said low pressure cylinder to open the by-pass, means ior loading the valve to 12 a predetermined opening pressure, and means operatively connected to the valve and adapted tor actuation by the suction pressure to open the valve against said loading pressure when the suction pressure exceeds the latter.

MALCOLM G. SHOEMAKER.

REFERENCES CITED The following references are of record in the the of this patent:

UNITED STATES PATENTS Number Name Date 1,561,118 Smith Nov. 10, 1925 1,607,531 Haight Nov. 16, 1926 1,649,297 Nelson Nov. 15, 1927 1,965,419 Lipman July 3, 1934 1,969,076 Hirsch Aug. 7, 193% 1,983,550 Lee Dec. 11, 1934 2,013,167 Musto Sept. 3, 1935 2,165,741 Wolfert July 11, 1939 2,319,502 Gould May 18, 1943 FOREIGN PATENTS Number Country Date 342,188 Great Britain Jan. 29, 1931 

